Mate tin plate



T. L sl-los MATTE TIN PLATE Filed Feb- 1962 Dec- 1964 IN TOR TOM L. SHOEMA/(ER By 294% ,ww W

United States Patent M This invention relates to an improved tin plate of dull appearance, i.e., matte plate, which is resistant to bake darkening after chemical treatment.

Matte tin plate has heretofore been made by the same process as bright plate except that the former is not heated after coating while the latter is heated to a temperature above the melting point of tin to cause refiowing or brightening of the tin coating. Both forms of tin plate require a surface treatment, usually in an aqueous solution of chromates, to impart resistance to discoloration in storage. Matte plate so treated, however, shows a pro nounced darkening on. baking incident to lacquering and lithographing. This is objectionable particularly in cases where portions of the sheet surface are'left un-inked to provide a light background for a lithographed design.

I have invented a novel matte tin plate which, though carrying the usual chemical surface film for prevention of storage discoloration, is not subject to bake darkening. I have discovered that this darkening results from a reaction between the chemically deposited film and the tin coating of matte plate as ordinarily made which carries a surface of amorphous tin ovide formed in air at atmospheric temperature or at temperatures below about 266 F. I overcome this by forming a crystalline tin oxide surface layer on the tin before chemical treatment. Thus the manufacture of my improved product involves merely heating the matte plate immediately after coating to a temperature above 266 F. but below the melting point of tin, to cause formation of a crystalline oxide instead of the usual amorphous oxide. This crystalline oxide is formed more rapidly at still higher temperatures. At commercial tin line speeds, a minimum temperature of 400 F. is required to form suflicient oxide for the purpose. After oxidation, the strip is given the usual chemical treatment.

A complete understanding of the invention may be obtained from the following detailed description and explanation which refer to the accompanying drawing, the single figure of which is a greatly enlarged section through a sheet of my improved matte plate.

Referring in detail to the drawing, a base sheet 10 of low-carbon steel of tin-plate gage, is provided by known processes with layers 11 of electrolytically deposited tin on both surfaces thereof. On emergence from the conventioual electrolytic tin-plating apparatus, the base strip, in clean dry condition, is immediately heated in air to a temperature above 400 F. but below the melting point of tin, for a few seconds, thereby forming on the surfaces of each of the tin layers 11 a film of crystalline tin oxide 12. Thereafter the coated base is chemically treated to form on top of the tin oxide films 12, films 13 of chromium oxides which protect the product from storage discoloration. The exact nature of the chemical film is not fully determined.

A more complete explanation of the method of making my improved matte product will appear from the following description of a typical example.

A continuous strip of tin plate is produced in the conventional manner by electroplating tin on a base oflowcarbon steel. After being electroplated, the strip is dried and then heated continuously to a temperature of about 430 F. by electrical-resistance heating. For this purpose sixty-cycle alternating current is supplied to the strip by two conductor rolls spaced 66 feet apart. The total time of heating is about 5 seconds at the strip speed of about 13 feet per second and the strip temperature is increased almost linearly to a maximum of about 430 F. The temperature of the strip is then immediately lowered by passing it through a water-quenching tank in which the second conductor roll is immersed. The strip is next passed between anodes immersed in an aqueous solution containing about 24 grams of sodium dichromateper liter, adjusted to a pH of about 4.7, at a temperature of about 125 F. The strip is made cathode in the solution and an electric current of about 52 coulombs per square foot of strip is passed between the strip and the anodes. After emerging from the treating solution, the

rip is rinsed with cold water, dried with steam, and

electrostatically oiled.

Subsequent analysis of the treated strip indicated the presence of 0.65 milligram of chromium (calculated as metallic chromium) per square footof strip surface. Des spite the presence of a deposit containing this quantity of chromium, however, the surface of the strip did not darken when baked at lacquer-curing temperature (375 F.) for thirteen minutes; in fact, the surface whiteness increased slightly. Another section of the same strip was treated in the same manner as described above, except that the heating step was not included. An equal quantity of chromium was deposited by the electrochemical treatment; but after the strip was baked, the total reflectance from its surface was 10% less than that from the surface of the strip processed by the method of my invention.

The duration of the heating step prior to electrochemical treatment may be varied from- 0.1 to 10 seconds at the maximum temperature, depending on the rate of heating and the maximum temperature attained. The heat may be applied by any convenient method consistent with the speed of the strip through the plating line. All electrolytic tin-plating lines include a melter for flow-brightening the tinsurface when a bright product is desired. In present practice, the melter is not used during the production,

of matte tin plate. The product of my invention may therefore be made commcrciallyby merely operating the melter, utilizing either resistance or induction heating or both, with a slightly reduced electric current to heat the strip to a final temperature approaching but below the melting point of tin.

It will be evident from the foregoingthat my invention or modification therein which may be made without de- References Cited in the file of this patent p 5 UNITED STATES PATENTS 2,450,508 Glock Oct. 5, 1948 2,687,994 Russell Aug. 31, 1954 2,820,747 Fredrickson Jan. 21, 1958 2,974,091

Neish Mar. 7, 1 961 Patented Dec. 8, 1964 

